ARTICLE

Multi-decadal establishment for single-cohort Douglas-fir forestsJames A. Freund, Jerry F. Franklin, Andrew J. Larson, and James A. Lutz

Abstract: The rate at which trees regenerate following stand-replacing wildfire is an important but poorly understood processin the multi-century development of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and western hemlock (Tsuga heterophylla(Raf.) Sarg.) forests. Temporal patterns of Douglas-fir establishment reconstructed from old-growth forests (>450 year) havegenerated contradictory models of either rapid (<25 year) or prolonged (>100 year) periods of establishment, while patterns oftree establishment in mid-aged (100 to 350 year) forests remains largely unknown. To determine temporal patterns of Douglas-firestablishment following stand-replacing fire, increment cores were obtained from 1455 trees in 18 mature and early old-growthforests in western Washington and northwestern Oregon, USA. Each of the stands showed continuous regeneration of Douglas-fir for many decades following initiating fire. The establishment period averaged 60 years (range: 32–99 years). These resultscontrast both with the view of rapid (one- to two-decade) regeneration of Douglas-fir promoted in the early forestry literature andwith reports of establishment periods exceeding 100 years in older (>400 year) Douglas-fir–western hemlock stands. These resultshave important implications for management designed to create and promote early-seral forest characteristics.

Key words: Douglas-fir, tree establishment, canopy closure, age structure, stand development, early-seral, single-cohort.

Résumé : Le taux auquel les arbres se régénèrent a la suite d’un feu sévère est un processus important, mais mal compris, dudéveloppement multiséculaire des forêts composées de douglas de Menzies (Pseudotsuga menziesii (Mirb.) Franco) et de pruche del’Ouest (Tsuga heterophylla (Raf.) Sarg.). Le patron temporel d’établissement du douglas de Menzies reconstitué a partir de vieillesforêts (> 450 ans) a produit des modèles contradictoires de période d’établissement rapide (< 25 ans) ou prolongée (> 100 ans)alors que le patron d’établissement des arbres dans les peuplements âgés de 100 a 350 ans est peu connu. Pour déterminer lepatron temporel d’établissement du douglas de Menzies a la suite d’un feu sévère, des carottes ont été prélevées sur 1455 arbresprovenant de 18 forêts matures et surmatures établies dans l’ouest de l’État de Washington et dans le nord-ouest de l’Oregon, auxÉtats-Unis. Un établissement continu de la régénération de douglas de Menzies pendant plusieurs dizaines d’années a été observéa la suite du feu a l’origine de chaque peuplement. La période moyenne d’établissement était de 60 ans (de 32 a 99 ans). Ces résultatssont différents de ceux véhiculés dans la vieille littérature forestière soit une régénération rapide (10 a 20 ans) du douglas de Menzieset de ceux mentionnés dans des rapports qui évaluent la période d’établissement a plus de 100 ans dans les vieux peuplements(> 450 ans) de douglas de Menzies et de pruche de l’Ouest. Ces résultats ont des implications importantes pour l’aménagement visanta créer et a promouvoir des caractéristiques forestières de début de succession. [Traduit par la Rédaction]

Mots-clés : douglas de Menzies, établissement d’arbres, développement des peuplements, début de succession, structure d’âge.

IntroductionTemporal patterns of tree regeneration following stand-

replacement disturbances strongly influence subsequent forestdevelopment (Ashton 1976; Foster et al. 1997; Cooper-Ellis et al.1999; Franklin et al. 2002). The initial period of tree establishmentfollowing high-severity disturbance is also significant, because itrepresents a stage in forest development during which trees donot exert strong competitive controls on other life forms, provid-ing essential habitat for many early-successional species (Swansonet al. 2011). Patterns of post-disturbance tree regeneration con-tinue to be of great interest to ecologists and managers who seekto understand ecosystem development to guide forest manage-ment and conservation of biological diversity (Franklin et al. 2002;Lindenmayer and Franklin 2002).

Post-fire tree establishment in Douglas-fir (Pseudotsuga menziesii(Mirb.) Franco) forests is a starting point for modeling changes instructure, function, and composition over multi-century develop-mental sequences (Huff 1995; Franklin et al. 2002; Zenner 2005;Larson et al. 2008). Douglas-fir–western hemlock (Tsuga heterophylla

(Raf.) Sarg.) forests in the Pacific Northwest experience large in-frequent wildfires, which typically include extensive areas ofstand-replacement severity (Agee 1993). The rate and density oftree establishment after stand-replacement wildfire is hypothe-sized to explain multiple alternative successional pathways (Donatoet al. 2012). For example, forests regenerating at high densitiesmay undergo canopy closure more rapidly leading to an intenseself-thinning phase, while stands that gradually re-establish atlower densities may not experience canopy closure and subse-quent self-thinning mortality (Franklin et al. 2002; Lutz andHalpern 2006; Halpern and Lutz 2013). Knowledge of the rate anddensity of tree establishment in naturally regenerated foreststherefore provides a basis for testing and refining conceptualmodels for forest succession and structural development.

Early 20th century forest researchers proposed a model of rapidand dense Douglas-fir establishment following stand-replacementwildfire (hereafter the “traditional model”). This early view ofDouglas-fir cohort establishment was strongly influenced by ob-servations of tree establishment following the 1902 Yacolt and

Received 30 December 2013. Accepted 18 April 2014.

J.A. Freund and J.F. Franklin. School of Environmental and Forest Sciences, College of the Environment, University of Washington, Box 352100,Seattle, WA 98195-2100, USA.A.J. Larson. Department of Forest Management, College of Forestry and Conservation, The University of Montana, Missoula, MT 59812, USA.J.A. Lutz. Quinney College of Natural Resources, Utah State University, Logan, Utah, UT 84322-5230, USA.Corresponding author: James A. Freund (e-mail: jafchen@uw.edu).

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Can. J. For. Res. 44: 1068–1078 (2014) dx.doi.org/10.1139/cjfr-2013-0533 Published at www.nrcresearchpress.com/cjfr on 28 April 2014.

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1910 Cispus fires in the southern Washington Cascades, and ofregenerating harvested stands in the same region (Hofmann 1924;Herring and Greene 2007). Julius Hofmann (1924), one of the ear-liest forest researchers investigating Douglas-fir regeneration,commented, “The main feature of interest found on the [Yacolt]burn was the good stand of young growth which almost uniformlycovered the area and consisted of the same species as those whichmade up the burned forest.” In an early synthesis paper on suc-cession and structural development of Douglas-fir forests, Munger(1940) concluded that Douglas-fir regeneration usually occurswithin a decade after the stand-initiating disturbance. The collec-tive observations of early forest scientists clearly pointed towardsa general model of rapid, dense establishment of the Douglas-fircohort following stand-replacement disturbance.

Some modern reconstructions provide evidence that supportsthe traditional model of rapid and dense Douglas-fir establish-ment. A detailed reconstruction of Douglas-fir establishment inan old-growth stand in the southern Washington Cascades (Winteret al. 2002a) provides some of the most compelling evidence, withthe establishment phase completed within 21 years. The tradi-tional model of rapid establishment remains well represented inthe current conceptual framing for natural Douglas-fir foreststructural development: Franklin et al. (2002) describe the initial,cohort-establishment stage as lasting for around 25 years, althoughthey do acknowledge potential for extended establishment.

Studies of old-growth (>400 years) forests beginning in the1970s led to the development of an alternative model in whichDouglas-fir establishment is protracted, lasting one to several cen-turies. Franklin and Hemstrom (1981) identified a >150 year periodof Douglas-fir establishment in the central Oregon Cascades.Tappeiner et al. (1997) reconstructed establishment periods fromseveral forests in coastal Oregon with establishment in 70% of thesites extending over >100 years (maximum of 364 years). A subse-quent study by Poage and Tappeiner (2002) also reported a pro-longed establishment period for Douglas-fir with a mean of 174and a maximum of 430 years. Poage and Tappeiner (2002) furthersuggested that short periods of establishment (one to two de-cades) are anomalous, and that prolonged periods are most com-mon (Poage et al. 2009).

In this study we examine the duration of Douglas-fir estab-lishment in 18 mature (100 to 200 year) and early old-growth(200 to-300 years) stands in western Washington and north-western Oregon that regenerated naturally following singlestand-replacement wildfires. Forests included in this study aregenerally older than the regenerating burns studied by Hofmann(1924), Munger (1930) and Isaac (1943), but are significantly youngerthan the (>400 year old stands analyzed by Tappeiner et al. (1997),Poage and Tappeiner (2002), Winter et al. (2002a, 2002b). Our pur-pose is to expand knowledge of temporal patterns of Douglas-firregeneration following wildfires in naturally-regenerated standsthat are younger than most previously studied. We pose two ques-tions: (1) Is the length of establishment of Douglas-fir in thesemature forests supportive of the “traditional model” of rapid es-tablishment or is it more comparable to prolonged periods ob-served in previous studies of old-growth? (2) How variable is theperiod of establishment and can it be explained by differences inlocal environment or current structural attributes?

Methods

Study areaDouglas-fir–western hemlock forests occur at low and middle

elevations in western Washington and northwestern Oregon(Franklin and Dyrness 1988). Large, infrequent (250 to 450 years)wildfires constitute the dominant disturbance regime in the West-ern Hemlock zone (Agee 1993), although in some parts of the regionthere is increasing evidence for a more diverse fire regime (Tepley2010), and large windstorms and volcanic eruptions can also occur

(Harcombe et al. 2004; Yamaguchi 1986). Over the last millen-nium, large wildfires in this region have created numerous ageclasses of forests dominated by Douglas-fir, western hemlock, andother conifers (Franklin and Dyrness 1988; Agee 1993, 1998; Ageeand Krusemark 2001). Many of these originated following a singlefire event, with little evidence of subsequent disturbance of suffi-cient size or intensity to allow for establishment of relatively shade-intolerant Douglas-fir (Isaac 1949; Herman and Lavender 1990).

Study sitesThe forests sampled were distributed between 47.95 and 44.27 N

latitude along the western slope of the Cascade Range, exceptfor one stand located in the eastern Olympic Mountains Washing-ton (Table 1) (Fig. 1). Forests occurred on relatively moist sitesand are classified by the Tsuga heterophylla/Polystichum munitum,Tsuga heterophylla/Polystichum munitum/Oxalis oregana, and the Tsugaheterophylla/hododendron macrophyllum/Gaultheria shallon plant associ-ations within the Western Hemlock Zone (Franklin and Dyrness1988). One site (Huckleberry) occurred in the cooler, moister Pa-cific Silver Fir Zone, classified under the Abies amabilis/Vacciniumalaskaense plant association (Franklin and Dyrness 1988). The cli-mate is maritime with cool wet winters and warm dry summers(Table 1). Common tree associates of Douglas-fir include westernhemlock, western redcedar (Thuja plicata Donn ex D. Don), westernwhite pine (Pinus monticola (Dougl.), Pacific silver-fir (Abies amabilis(Dougl. ex. Loud) Dougl. ex Forbes), Pacific yew (Taxus brevifolianutt.), noble fir (Abies procera), big-leaf maple (Acer macrophyllumPursh), and Pacific dogwood (Cornus nutallii Aud.).

Site selectionCandidate sites were carefully examined and excluded if

there was evidence of post-initiating disturbance (fire or wind)that could have facilitated subsequent establishment of Douglas-fir. Fine-scale disturbances (root rot, bark beetles, or wind) thataffected individual trees or small clusters of trees (creating canopygaps) were evident in most stands and were not used as criteria forrejection.

Sample sites were of two types: (i) early old-growth stands sam-pled using multiple temporary plots and (ii) mature stands sam-pled using permanent plots that were established decades beforethis study (http://andrewsforest.oregonstate.edu/). Sites sampledwith temporary plots (n = 9 sites) were established following areview of fire-history studies (Morrison and Swanson 1990; Agee1991; Impara 1997; Weisberg and Swanson 2003), stand age-classmaps, and conversations with several ecologists familiar with theregion (personal communications with Ken Bible, Rolf Gersonde,Scott Gremel, Charles Halpern, Jan Henderson, Robin Lesher, Rob-ert Van Pelt), and extensive reconnaissance.

Nine additional sites were selected from an established net-work of permanent sample plots (hereafter, PNW-PSP) (Acker etal. 1998). Plots were selected from this network based on approx-imate age (100–200 years), dominance by Douglas-fir, and lack ofevidence of disturbances post canopy closure. Forests originatedfrom stand-replacing disturbances, varied in age, and were dis-tributed over a broad latitudinal range (Table 1).

Several sites were located close enough to have regeneratedafter the same wildfire. Drift Creek, Osprey, Skynard Hill, andCedar Flats lie within 10 km of each other in the central LewisRiver Valley in the southwest Washington Cascades and evidentlyestablished after large-scale fire ca. 1800. Stands WR04, WR05,WR90, and Meditation Rock lie within 8 km of each other in thePanther Creek Division of the Wind River Experimental Forest. Al-though these sites may be considered pseudoreplicates (Hurlbert1984), given the variability in fire severity and site conditions withina large wildfire it is unlikely that the two samples represent the“same” fire. Furthermore, understanding the variability created bysingle large-scale disturbances remains valuable (Turner et al. 1997;Larson and Franklin 2005).

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Field procedures for temporary plotsSampling was conducted between June and September during

2006–2010. In each stand, six 0.2 ha circular plots were establishedat 100 m intervals along a linear transect. Transects were initiatedfrom a random starting point after stand shape, boundaries, andtransect azimuth were determined. Within each plot, all trees≥5 cm dbh were identified by species, and diameters were mea-sured 1.37 m above the base of the tree. Douglas-fir was an abun-dant component of the overstory in each site, indicating thatDouglas-fir dominated the tree establishment phase (Table 1). De-pending on Douglas-fir density in each circular plot, 6–10 liveDouglas-fir were subjectively chosen for aging to represent thefull range of size classes (and presumably ages). To determine thatthe size distribution of aged trees was representative of the largermeasured population, we used a series of Kolmagorov–Smirnov(K–S) tests; all tests confirmed that there were no significant dif-ferences among cored tree diameter distributions and the remain-ing Douglas-fir diameter distribution.

All cores were taken at breast height (1.37 m). Although corestaken closer to the ground surface may improve the accuracy ofaging, they are more difficult to obtain and more likely to includebutt rot. Each core was visually inspected to determine if pith wasreached. If pith was not reached and the core did not contain themajority of inner-most rings, a second core was taken. Time andlabor constraints limited additional coring beyond two cores pertree. If a suitable core was not obtained after two attempts, analternate tree of similar diameter was sampled. Cores were placedin paper straws, labeled, and stored in a waterproof container forsubsequent laboratory analysis.

Field procedures for permanent plotsBased on existing diameter data for Douglas-fir, a stratified ran-

dom sample was used to select trees for aging. Diameter distribu-tions were stratified using 15 cm diameter bins, and 40%–60% ofindividuals per diameter bin were selected for coring. Methods forobtaining increment cores were the same as described for tempo-rary plots.

Laboratory methods for aging increment coresAll increment cores were prepared using standard dendro-

chronological techniques (Stokes and Smiley 1968). All cores weremounted on grooved wooden boards and sanded three times atprogressively finer grit (100, 220, and 400). Cores with narrowgrowth rings required additional sanding at 600 and 800 grit.Annual rings were counted with a microscope (3×); cross-datingwas not attempted because it was not relevant to the study objec-tives. While cross-dating is essential for determining accurate pithdates for some species and growing conditions, we evaluated thatit was not necessary for the Douglas-fir on moderately productivesites in the current research. A study by Weisberg and Swanson(2001) showed that for Douglas-fir samples, pith dates determinedby careful laboratory ring counts on well prepared surfaces werevery close to (mean difference of 1.24 years) pith dates determinedby cross-dating the samples. To estimate the number of ringsmissing in cores that lacked a pith, we used a transparent tem-plate containing concentric circles that best fit the growth patternpresent in the inner core (Applequist 1958). We examined height–growth curves for Douglas-fir in conjunction with a site produc-tivity table (McArdle et al. 1961) to estimate age to core height.Seven to nine years were added to each site for the core heightcorrection. The ring count, estimate of rings to pith, and estimateof age to core height were summed to obtain an estimated totalage for each tree. Summed ages represent age at sample year.

AnalysesThe following summary statistics were calculated for aged

Douglas-fir trees at each site: age range (including 95% confidenceintervals [CI]), duration of establishment for 90% of the aged trees(including 95% CI), minimum age, maximum age (as an estimateof cohort initiation), and standard error of age, to address ques-tion 1. A frequency distribution was then created for each site;10 year age bins were used to facilitate comparison with previousstudies of Douglas-fir age structure (Klopsch 1985; Stewart 1986; Huff1995; Tepley 2010) and to assess the pace of establishment by decade.

Table 1. Site characteristics for sampled stands.

SITE Lat. Long.Elevation(m)

Max. standage (year)

Douglas-firdensity (%)

Douglas-firbasalarea (%)

Annualprecip.(mm)

July meanprecip.(mm)

Jan MeanPrecip(mm)

July maxtemp. (°C)

Jan mintemp (°C)

Growingseason meanprecip. (mm)

Olympic National Park, Washington1 Sol Duc 47.95 −123.81 687 317 31 71 3622 61 483 20.3 −1.2 80.0Cedar River Watershed, Washington2 Huckleberry 47.31 −121.52 799 328 16 25 2584 62 397 21.0 −2.9 74.7Mt. Rainier National Park, Washington3 AX15 46.75 −121.82 1024 183 27 42 2099 44 318 22.4 −3.4 62.64 TB13 46.74 −121.84 1018 213 52 31 2004 43 296 22.2 −3.6 61.15 Ohanapecosh 46.74 −121.55 670 296 25 47 1974 35 306 23.9 −3.9 46.61Gifford Pinchot National Forest, Washington6 Cedar Flats 46.11 −122.01 400 191 2 64 2972 45 455 25.2 −1.9 64.57 Drift Creek 46.03 −122.09 324 193 54 81 2990 46 455 24.6 −1.2 65.98 Osprey 46.03 −122.08 318 190 37 83 2995 46 456 24.9 −1.1 66.19 Skynard 46.02 −122.08 379 193 31 82 3339 51 526 24.0 −1.4 71.110 WR90 45.88 −121.90 715 171 33 86 2704 27 450 23.2 −2.7 43.811 WR05 45.84 −121.87 365 177 45 88 2506 21 399 26.9 −2.0 39.712 WR04 45.83 −121.87 315 168 35 91 2508 21 397 26.8 −2.0 40.113 Meditation Rock 45.49 −121.48 823 192 87 98 1226 22 194 22.5 −4.1 21.814 Goshawk 45.80 −121.97 494 114 85 98 1081 25 190 25.3 −3.7 17.4Mt. Hood National Forest, Oregon15 MH123 45.31 −121.91 605 130 46 91 2259 45 325 23.2 −2.6 63.2Willamette National Forest, Oregon16 Bagby 44.94 −122.17 650 297 27 66 2123 31 301 24.1 −2.4 51.717 Breitenbush 44.79 −121.90 760 326 32 77 1997 33 304 24.7 −3.8 45.518 RS26 44.27 −122.18 990 174 39 93 2130 28 288 27.5 −1.5 46.8

Note: Site numbers correspond to Fig. 1. Climate data from PRISM (Daly et al. 2002).

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Cumulative age distributions (as cumulative proportion of es-tablishment) were developed to compare establishment patternsamong sites (question 2). Although the total duration of tree es-tablishment was of primary interest, the period representing 90%of individuals was also a useful metric for assessing the rapidity ofconifer establishment.

Cumulative age distributions among sites were quantitativelycompared using nonlinear regression. Data were fit to sigmoidalmodels using SigmaPlot Version 11.0 (Systat Software, Chicago, Illi-nois, USA). Parameter estimates representing time to 50% establish-ment and slope were evaluated to quantify inter-site variation.

Pearson correlation analyses were performed to explore whetherassociations existed between environmental variables and durationof establishment. Correlation matrices were created for extrinsicenvironmental variables of latitude, elevation, aspect, mean annualprecipitation, mean July precipitation, mean January precipita-

tion, mean growing season precipitation, maximum July temper-ature, and minimum January temperature. Pearson correlationswere also performed on some stand variables of Douglas-fir toexplore biotic controls on establishment.

Maximum age in each stand, percentage of the Douglas-firpopulation aged, percentage Douglas-fir density, and percentageDouglas-fir basal area were used as biotic correlates. Pearson cor-relation analyses were also performed using the parameter esti-mates (X0, b) derived from the sigmoid regression with the sameset of environmental and stand variables to assess the shape of thecurve of tree establishment.

ResultsAge ranges

Duration of establishment averaged 60 years (95% CI of 50–70 years) and ranged from 32 to 99 years (Fig. 2). Establishment

Fig. 1. Map of Washington and Oregon with study locations marked with a yellow stars. Dark grey portion represents the crest of CascadeMountains.

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periods exceeded 75 years for only three stands (Table 2). Seven-teen percent of stands established within 40 years, 40% estab-lished within 50 years, and 83% established within 75 years. Moststands achieved 90% establishment of Douglas-fir several decadessooner (mean of 35 years; 95% CI of 28–42 years) than full establish-ment. Duration of 90% establishment ranged from 19–65 years, with67% of stands within 40 years and 83% of stands within 50 years. Attwo sites, minimum and maximum ages differed by 82 (Medita-tion Rock) and 115 years (WR05); summary statistics are reportedincluding and excluding these individuals, because the trees thatestablished later represent individuals that established in canopygaps following forest canopy closure. Canopy gaps were visuallyinspected during sampling and represented areas where severalDouglas-fir in early stages of decay were present.

Age distributionsSites display broadly comparable cumulative age distributions

regardless of the century when establishment began (Fig. 3). Mostof the variability among cumulative age distributions is found inthe 12 oldest sites (Table 2); the greatest variability in age distri-butions is found in 6 sites with maximum ages between 183 and213 years (SE 0.88–3.43). Sigmoid curves were well fit to the cumu-lative age distributions with r2 values in the range 0.94–0.99(Fig. 4). Parameter estimates (X0, b) were assessed to compare thepoint at which each site experienced a steep increase in slope. Lowvalues for b with corresponding low values of X0 appear to indi-cate rapid establishment and a shorter establishment duration.Increasing values for b with corresponding increases in X0 appearto represent a process characterized by steady and gradual estab-

Fig. 2. Histograms of the distribution of Douglas-fir ages in 10 year bins. Panels are arranged from youngest to oldest sites. WRO5 has 9 treesthat established in a canopy gap. Meditation Rock has one individual that established in a canopy gap following canopy closure.

Goshawk

0.0

0.2

0.4

0.6

0.8

1.0

MH123

0.0

0.2

0.4

0.6

0.8

WR04

0.0

0.2

0.4

0.6

0.8

WR90

Prop

ortio

n of

Age

d D

ougl

as-fi

r

0.0

0.2

0.4

0.6

0.8

RS26

0.0

0.2

0.4

0.6

0.8

WR05

Age (years)0 40 80 120 160 200 240 280 320

0.0

0.2

0.4

0.6

0.8

AX15

Osprey

Cedar Flats

Meditation Rock

Skynard

Drift Creek

Age (years)0 40 80 120 160 200 240 280 320

TB13

0.0

0.2

0.4

0.6

0.8

1.0

Ohanapecosh

0.0

0.2

0.4

0.6

0.8

Bagby

0.0

0.2

0.4

0.6

0.8

Sol Duc

0.0

0.2

0.4

0.6

0.8

Breitenbush

0.0

0.2

0.4

0.6

0.8

Huckleberry

Age (years)0 40 80 120 160 200 240 280 320

0.0

0.2

0.4

0.6

0.8

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lishment. The highest values for b and X0 appear to represent anestablishment process characterized by slower more gradual es-tablishment. Parameter estimates derived from the sigmoidal re-gressions did not yield any statistical association with eitherenvironmental or stand level variables (P > 0.05) (Table 3).

Duration of establishment did not correlate with geographiclocation. The two shortest establishment periods (32 and 33 yearsat WR04 and Cedar Flats) occurred in the southern WashingtonCascades and represented different wildfires. The longest estab-lishment periods were broadly distributed over the geographicrange of sites: 88, 94, and 99 years at Drift Creek, Breitenbush, andSol Duc, respectively (Fig. 2). In addition, the second shortest andthird longest establishment periods occurred in the same rivervalley (several kilometers apart) presumably following the samewildfire ca. 1800 (32 years at Cedar Flats and 88 years at DriftCreek); both of these forests also represent the most productivesites (site index I; Isaac [1949]).

Pearson correlations yielded no significant statistical associa-tions between establishment duration and elevation, latitude, as-pect, maximum stand age, percent of the Douglas-fir populationaged, percent Douglas-fir density, percent Douglas-fir basal area,mean annual precipitation, mean July precipitation, mean Janu-ary precipitation, mean growing season precipitation, maximumJuly temperature, and minimum January temperature (P > 0.05).

DiscussionThe majority of Douglas-fir forests sampled in this study estab-

lished within 75 years after the initiating wildfire and alwayswithin 100 years. Duration of establishment did not differ as afunction of the period during which stands established, nor did itvary with any of the environmental or stand structural variablestested in the correlation analysis. Therefore, we infer patterns ofsingle cohort establishment following high-severity wildfire wererelatively similar, at least in the stands that we examined. More-over, we conclude that stand aspect and the regional climaticgradient covered in this study do not directly influence thepattern Douglas-fir establishment. We find it interesting that

establishment periods for Douglas-fir regeneration followingnon-stand replacement fire in the central Oregon Cascades(Tepley 2010) were found to be nearly as long as those observedin our study of stand replacement fire. These findings bolsterour interpretation of a consistent range of establishment rateand suggest that regardless of high or mixed-severity wildfire,cohorts of Douglas-fir in the western Cascades establish with asimilar time course (Fig. 5).

Our study appears to provide the only significant data set onsingle cohort establishment patterns of Douglas-fir from matureage classes of naturally regenerated forests. A few data are avail-able from other comparable stands, a roughly 200 year old standat Pack Forest near Mount Rainier (Kuiper 1994) with an age rangefor Douglas-fir > 100 years but with 70% of the stems establishedwithin 50 years, and a roughly 200 year old stand in the westernOlympic Mountains (Huff 1995), indicating complete Douglas-firestablishment within 75 years. Zenner (2005) found a 46 year agerange for Douglas-fir establishment in a mature single cohortDouglas-fir stand in the southern Oregon Cascade Range. Yanget al. (2005) reconstructed early-successional conditions using aerialphoto interpretation and a Chapman–Richards growth function andreported establishment periods of 40–50 years in plantations locatedin the western Cascades of Oregon, providing further support for ourinterpretation of a several decade period of early-seral forest condi-tions.

Comparisons with conceptual models of Douglas-fir standestablishment

Douglas-fir establishment durations in this study are not wellrepresented by any of the previously developed conceptual mod-els. Rates of establishment observed in mature and early old-growth stands in this study contrast with the traditional model ofrapid stand establishment inferred from the observations of earlyresearch foresters, such as Munger (1940), Hofmann (1924), andIsaac (1943). However, we note that their focus was actually onestablishment of sufficient Douglas-fir regeneration to ultimatelyproduce a fully-stocked stand and not on the temporal duration of

Table 2. Summary statistics for Douglas-fir ages.

Maximumage (year) Study area Plot type

Single-cohortestablishmentduration (year)

Years to 90%establishment n

Mean pithcorrection(SD)

Douglas-fir(ha)

MeanDouglas-firDBH (SD)

Douglas-firbasalarea (m3·ha−1)

114 Goshawk Permanent 60 30 411 2 (2) 414 39.5 (15.0) 58.0130 MH123 Permanent 47 22 161 3 (1) 289 51.6 (13.2) 64.5168 WR04 Permanent 45 32 24 4 (1) 159 33.5 (30.4) 67.0171 WR90 Permanent 75 32 43 4 (2) 163 72.8 (20.1) 72.7174 RS26 Permanent 75 43 89 3 (2) 302 58.7 (20.6) 91.7177 WR05 Permanent 32 19 56 2 (2) 195 63.4 (26.6) 72.2177 WR05* 147 135 65 — — — —183 AX15 Permanent 44 19 82 3 (2) 173 47.1 (13.9) 32.8190 Osprey Temporary 38 34 48 4 (3) 85 93.6 (33.4) 65.3191 Cedar Flats Temporary 33 19 31 4 (3) 62 98.3 (25.2) 50.0192 Meditation Rock Permanent 67 48 135 3 (2) 212 61.68 (19.23) 64.6192 Meditation Rock** 149 48 136 — — — —193 Skynard Hill Temporary 47 33 52 4 (2) 65 101.4 (26.7) 56.1193 Drift Creek West Temporary 88 51 42 4 (2) 87 91.9 (38.1) 67.4213 TB13 Permanent 69 25 50 3 (2) 108 70.4 (28.5) 48.8296 Ohanapecosh Temporary 61 49 32 5 (3)) 72 81.4 (24.7) 40.7297 Bagby Temporary 48 19 42 4 (2) 93 87.6 (16.0) 57.5317 Sol Duc Temporary 99 58 37 3 (2) 118 91.0 (31.8) 86.7326 Breitenbush Temporary 94 65 39 4 (2) 108 95.7 (29.3) 84.5328 Huckleberry Temporary 64 34 71 2 (2) 74 61.9 (12.8) 23.2

Mean Values 60 +/− (10.1) 35 +/− (7.1) — — — — —(95% CI)*** 50–70 years 28–42 years — — — — —

*Includes 9 individuals that established in a canopy gap.**Includes 1 individual that established in a canopy gap.***Means and 95% CIs do not include trees establishing in gaps. Pith correction represents the mean number of years added to cores from each site with the standard

deviation in parentheses. Stand data for Douglas-fir trees reported in the final three columns. DBH represents diameter at breast height.

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tree seedling establishment or on the rate at which forest cano-pies closed. This may be why their accounts of successful Douglas-fir provide no indications that regeneration may continue formany decades following a stand initiating disturbance. Therefore,rates of establishment in this study provide a useful context to com-pare with previous theory and observations regarding Douglas-firestablishment.

Rates of establishment in this study also contrast with descrip-tions of protracted, low-density establishment in older (>400 yearold) forests. Douglas-fir regeneration in many older stands wasinterpreted as occurring over periods of more than 100 years(Hemstrom and Franklin 1982; Tappeiner et al. 1997; Poage andTappeiner 2002). Confidence intervals (95%) for Douglas-fir estab-lishment are between 104 and 237 years (n = 10) in Tappeiner et al.(1997) and Poage, and Tappeiner (2002) report 95% confidence in-

tervals for the mean of establishment at 134–213 years (n = 27). The95% confidence intervals for the mean of total establishment re-ported in this study (95% CI = 50–70 years, n = 18) clearly do notoverlap with these.

Rates of establishment observed in this study contrast signifi-cantly with the model of plantation forestry. Our results suggestthat persistence of open canopy conditions for several decades iscommon in natural forest development. In contrast, dense anduniform plantations managed for wood production are designedto achieve immediate establishment of fully-stocked stands, withcanopy closure occurring within a decade or less (Smith et al.1997).

A comparison of studies where Douglas-fir ages were used toreconstruct establishment duration indicates a complex suite offindings. In the 1970s scientists began their studies of Douglas-firestablishment by counting annual growth rings on cut stumps ofDouglas-fir trees after the stands had been harvested. Their find-ings of age ranges spanning 1–2 centuries were inconsistent withthe accepted wisdom of rapid Douglas-fir establishment and for-est canopy closure, which puzzled them (Franklin and Hemstrom1981). Subsequent studies of Douglas-fir ages in old-growth for-ests >400 years in Oregon also were interpreted as providing evi-dence of periods of establishment extending over 1 or more centuries(Stewart 1986; Tappeiner et al. 1997; Poage and Tappeiner 2002).Several hypotheses were proposed to explain these findings in-

Fig. 3. Cumulative patterns of establishment of Douglas-fir in sitesof increasing age (top to bottom panels). Black vertical lines areplaced at 25 years to compare with establishment rates outlined inthe Franklin et al. 2002 conceptual model. Black vertical lines arealso placed at 50 years to highlight establishment rates that lasttwice as long as or longer than suggested by the traditional model.

0.0

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GoshawkMH123WR04WR90RS26WR05

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AX15OspreyCedar FlatsDrift CreekSkynardMeditation RockTB13

Time Since First Establishment (yr)

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 1500.0

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OhanapecoshBagbySol DucBreitenbushHuckleberry

Fig. 4. Sigmoidal regression curves fit to cumulative agedistributions for each sample site.

Time Since First Establishment (yr)

0 20 40 60 80 100

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AX15TB13OhanapecoshBagbyBreitenbushHuckleberrySol DucDrift Creek WestSkynardOspreyCedar FlatsMeditation RockGoshawkRS26WR90WR04WR05MH123

Table 3. Parameter estimates for sigmoidal regression on cumulativeage distributions.

SiteModel form f = a/(1 + exp(−(x−x0)/b)) b x0

AdjustedR2 P

Goshawk 4.51 44.29 0.97 <0.001MH123 3.63 35.58 0.99 <0.001WR04 4.08 37.82 0.94 <0.001WR90 5.93 59.10 0.97 <0.001RS26 7.55 53.18 0.98 <0.001WR05 3.30 22.32 0.98 <0.001AX15 3.84 34.27 0.98 <0.001Osprey 7.66 20.63 0.97 <0.001Cedar Flats 2.89 20.15 0.96 <0.001Meditation Rock 5.35 36.28 0.99 <0.001Skynard Hill 5.02 31.93 0.95 <0.001Drift Creek 12.69 61.13 0.96 <0.001TB13 4.70 55.99 0.98 <0.001Ohanapecosh 7.52 35.03 0.97 <0.001Bagby 4.30 38.36 0.97 <0.001Sol Duc 7.57 65.56 0.97 <0.001Breitenbush 12.67 51.85 0.98 <0.001Huckleberry 5.10 41.35 0.99 <0.001

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cluding seed source limitations after the extensive fires of the late15th and early 16th centuries, competition from hardwood shrubsand trees, and creation of opportunities for further Douglas-firestablishment by subsequent partial stand replacement distur-bances. Some investigators examined the wide early growth ringsin old Douglas-fir and hypothesized stand establishment to haveoccurred at low initial stand densities (Tappeiner et al. 1997; Poageand Tappeiner 2002). Notably, these studies were conducted in areassouth and west of the study sites reported here and may reflect morefrequent fire history at low elevations in the Willamette Valley andcoastal ranges of Oregon than those of more northerly sites in Wash-ington. Shorter pulses of Douglas-fir establishment were ob-served in two studies in Washington State, lending support forthe original hypothesis of rapid forest establishment and standclosure (Yamaguchi 1986; Winter et al. 2002a).

Variation of study findings from old-growth forests couldrepresent several pathways of early succession (Donato et al.2012), or could be the result of other factors that obscure thedifference between overall age structure and establishment du-rations. For example, subsequent disturbance including partial,stand-replacement fires may lead to multiple cohorts and periodsof establishment (Tepley et al. 2013). Tepley’s (2010) meticulousstudy of wildfire disturbances and Douglas-fir tree ages in old-growth forests in the central Cascade Range of Oregon providesstrong evidence that at least some of the observed extended peri-ods of Douglas-fir establishment represent multiple cohorts thatestablished after partial stand-replacement fires. Another factorthat could contribute to interpretations about extended periodsof establishment relate to errors associated with counting ringson stumps in the field. Studies that reconstructed patterns of earlydevelopment from stump counts in harvested units almost invari-ably involved significant errors in ring counts, which would tendto blur evidence for distinct Douglas-fir cohorts (Weisberg andSwanson 2001; Tepley 2010). When ages of cross-dated samples of

old-growth Douglas-fir trees were examined in a recent study,field counts on cut stumps were conclusively found to be inaccu-rate and thus unsuitable for quantifying the duration of establish-ment (Tepley 2010). However, one study (Sensenig et al. 2013)reports that errors in counting annual rings does depend on ringsper centimeter increment, and that old-growth trees consistentlygrew at 10 rings per centimeter.

Different study objectives, sampling procedures, and recon-struction methodologies are likely causes for some of the vari-ability in establishment durations among multiple studies.Reconstructions of forests >500 years old require meticulous anal-ysis that includes cross-dating (Winter et al. 2002a, 2002b; Tepley2010), because evidence of disturbance and tree death and decaycan be masked by many centuries of development. Small-scaledisturbance events, which kill individual or small groups of can-opy dominants, can eliminate structural features needed for ac-curate reconstructions of early tree establishment conditions(Gray and Spies 1997; Lutz and Halpern 2006; Kane et al. 2011) oreven create entirely new cohorts. Stands that undergo gap forma-tion created by wind, root rots, or beetles may exhibit low densityrecruitment of Douglas-fir. Such gap recruitment of shade-intolerantspecies, such as Douglas-fir, over centuries could potentially create acomplex multi-modal age structure and create confusion regardingcontinuous establishment. Two sites included in this study (Medi-tation Rock, WR05) displayed recruitment of Douglas-fir, whichdeveloped in significant canopy openings after the initial cohorthad closed canopy. These trees were certified as true new recruitsby field observations, measurement of the snag population (datanot reported) and coring, providing evidence that Douglas-fir cansuccessfully regenerate in forests with multiple canopy gaps. Re-constructing initial periods of establishment in younger forests,as in this study, can be more accurate reconstructions of estab-lishment than in old-growth because: (1) extensive periods ofagent-based mortality, decomposition, or gap creation have not

Fig. 5. Conceptual model of single-cohort Douglas-fir establishment following high severity wildfire. The brown line represents a plantationsystem with synchronous establishment and rapid canopy closure (within a decade). The red line represents a developmental pathway inwhich establishment extends more than two decades culminating in a closed-canopy state at 25 years as suggested by Franklin et al. (2002).The solid green line represents the establishment patterns reconstructed in this study. Dashed green lines illustrate the variation inestablishment duration (minimum and maximum values). The horizontal dotted line represents the time in years at which 90% of Douglas-firhave established. The box and whisker plots illustrate the variation in the duration of establishment for all trees and 90% of trees for all sites;horizontal lines represent means lower and upper boundaries representing the 25th and 75th percentiles, respectively. Whiskers representminimum and maximum values. (See online version for colour.)

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occurred in the forest and (2) higher quality age and ring data aremore easily obtained from the smaller trees found in young andmature stages of forest development.

That said, extended (>100 year) periods of establishment canoccur in Douglas-fir forests. Undoubtedly, variable rates of estab-lishment are influenced by the extent and behavior of the initiat-ing disturbance, presence or absence of surviving seed-bearingtrees, the presence and abundance of broadleaf tree and shrubspecies (Franklin and Hemstrom 1981). Periods of repeated largeintense wildfires might well have created areas without signifi-cant surviving Douglas-fir seed trees; such areas would initiallyundergo establishment of a low-density founding population, whichwould eventually provide a local seed source to fill in stands. More-over, repeated wildfires occurring before an initial cohort hasreached reproductive maturity may be a factor in extending es-tablishment periods, a phenomena that has been observed inother forest systems (Larson et al. 2013). Obviously, each sere willexhibit idiosyncratic patterns, which could include such a possi-bility, but the probability would be low and most logically occuron very dry sites within the Western Hemlock zone (Means 1982;Tepley 2010), on very frosty sites within the Pacific Silver Fir zone(Franklin and Hemstrom 1981), on sites with multiple fires at shortreturn intervals (Isaac and Meagher 1936; Huff 1995; Gray andFranklin 1997; Tepley 2010), and on subalpine sites (Franklin et al.1988).

Establishment and among site variationCumulative patterns of establishment are generally similar in

form, but the rate at which each stand established is variable(Fig. 4). Establishment rate for the majority of stands supports ageneral model of establishment where trees gradually but consis-tently establish until canopy closure occurs. The consistency withwhich the majority of stands follow a “steady” establishmentmodel likely reflects a combination of availability of seed sourcefrom surviving legacy trees and competition from shrub and hard-wood species (Yang et al. 2005). Some establishment rates withinthis data set also support alternative models. Two sites, WR05 andCedar Flats, exhibit establishment duration and cumulative agedistributions supporting a “rapid model” of tree establishment.The canopy seed bank described by Larson and Franklin (2005)could contribute to rapid and dense establishment following thestand initiating wildfire. A more “prolonged” model of establish-ment is also represented in this data set by the Breitenbush andSol Duc sites,and may be the result of founding trees that becomeestablished and provide future seed source. Prolonged establish-ment could also occur when stands experience multiple fireswithin the first few decades after stand replacement events (Grayand Franklin 1997).

Establishment rates can vary even on productive sites whensubjected to the same fire event, although we acknowledge thatfires were not dated in this study. The second shortest and thirdlongest durations for establishment occurred on productive sitesseveral kilometers apart in the same river valley following whatwe believe was the same wildfire event. Both Cedar Flats andDrift Creek are on productive sites and Franklin et al. (2002)hypothesized that productive sites would experience canopyclosure sooner than lower productivity sites. Larson et al. (2008)examined the influence of productivity on forest developmentand found that highly productive stands dominated by coniferspecies recover from disturbances more rapidly. However, thediffering rates of establishment at Cedar Flats and Drift Creeksupport the idea that relationships between productivity andstand development, can vary greatly.

Comparison of tree establishment in Douglas-fir with othermoist temperate forests

The multi-decade period of tree establishment in these Douglas-fir forests is long compared to the post-disturbance recruitment

of seral tree species in other moist temperate forests. Eucalyptusregnans (F. Muell) forests of Australia commonly regenerate anddevelop closed canopies rapidly (1–5 year) following severe wild-fire; by 40 years the forest is well into a thinning phase (Ashton1976). Quercus rubra–Acer rubrum forests of central Massachusetts,as well as other forests in the northeastern United States, aredominated by angiosperms and abundantly propagate throughsprouting also exhibit rapid regeneration and canopy closure fol-lowing high severity wind disturbances (Cooper-Ellis et al. 1999).

Management implicationsEarly-seral ecosystems develop on forested sites between a

stand-replacement disturbance and re-establishment of a closedforest canopy (Swanson et al. 2011). These ecosystems typicallyshare dominance among diverse plant life forms and include in-dividuals and groups of trees. Animal diversity is typically highand often includes species that are restricted to early-seral habitatspecialists. Providing for such ecosystems is emerging as an im-portant management goal on public lands (Franklin and Johnson2012), but the scientific knowledge base for planning and imple-menting such management is limited. Specifically relevant to thisstudy, little is known about how long such ecosystems persisted invarious forest types, including those occupying the Western Hem-lock Zone of the Pacific Northwest. This study provides evidencethat it may not be appropriate to replant in post-wildfire land-scapes where provision for earl- seral conditions is an objective.

The Douglas-fir establishment rates in naturally regeneratedforests reported in this study differed dramatically from indus-trial plantations. Typical plantations involve intensive site prepa-ration activities that focus primarily on planting sites at highstocking levels to achieve tree dominance and canopy closure assoon as possible (Smith et al. 1997; Curtis et al. 2007). Federalforest plantations share some characteristics of industrial planta-tions, however there is evidence that federal plantations can havemulti-decadal establishment rates (Yang et al. 2005). The differ-ence in timing of establishment between natural establishingstands and plantations has significant implications for many eco-system values. Therefore, managers seeking to mimic natural pro-cess will need to consider alternatives to dense plantations. Forexample, when objectives necessitate mimicking natural processes,several possibilities, including reliance on natural vegetation or cre-ating plantations that are less dense or more heterogeneous orboth, may be appropriate, especially when creation of high-qualityearly-seral conditions is a management objective. Plantation man-agement places a higher value on the rapid re-establishment offorest cover and generally ignores the ecological values associatedwith pre-canopy closure ecosystems. Policies within the NationalForest Management Act of 1976 mandate that each federal forestlands have targets and strategies to replant logged over or dis-turbed landscapes, yet this approach may be in total opposition tostated agency goals (e.g., restoration) and management objectives,which are aimed at mimicking natural processes. These policiesand associated regulations need to be addressed in specific waysto provide managers with flexibility to achieve stated goals. Ulti-mately, nuanced policies that recognize and provide for all suc-cessional stages are needed.

AcknowledgementsFunding provided by the Wind River Canopy Crane Research

Facility and the United States Forest Service, PNW research sta-tion. Data were collected through the Pacific Northwest Perma-nent Sample Plot Program (PNW-PSP), which is funded by theNational Science Foundation’s Long-Term Ecological ResearchProgram (DEB 08-23380), the US Forest Service Pacific NorthwestResearch Station, and Oregon State University. We thank KenBible, Matt Schroeder, Mark Creighton, and Annette Hamilton forlogistical support at the Wind River Canopy Crane Research Facil-ity. Nicholas Povak provided excellent statistical consultation.

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Robert Van Pelt and Alina Cansler provided assistance with maps.This manuscript was improved with the help of two peer review-ers. Linda Winter, Keala Hagmann, Derek Churchill, and LaurenUrgenson provided helpful reviews of this manuscript. Fieldworkand laboratory assistance was provided by Warren Childe, JohnFitzpatrick, Forest Heitpas, Patrick Mcguire, Alison Sigler, HeidiHume, and Drew Jensen. We thank Scott Gremel, Robin Lesher,and Jan Henderson for forest age class maps and information.

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Agee, J.K. 1991. Fire history of Douglas-Fir forests in the Pacific Northwest. In Wildlifeand Vegetation of Unmanaged Douglas-fir Forests. Edited by L.F. Ruggiero, K.B.Aubry, A.B. Carey, M.H. Huff. U.S. For. Serv. Gen. Tech. Rep. PNW-GTR-285.

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